This invention relates to an engine or vehicle exhaust measurement device, and more particularly, the invention relates to a compact direct vehicle exhaust flow measurement device (DVE), which may be easily packaged within a test cell while providing compensation for engine exhaust pulsations when needed.
The determination of engine or vehicle exhaust emissions during a test cycle is necessary to verify compliance with emission standards. To quantify the products of combustion correctly either the exhaust flow has to be known or an alternative method has to be used. A common method is the CVS (Constant Volume Sampling) principle, which has been used for many years in the industry and is a widely used standard. A CVS system samples the total amount of exhaust gas expelled by the engine or vehicle and dilutes it with air. The total flow as a sum of exhaust gas and dilution air is kept virtually constant. A constant sample is taken at a point far enough downstream of the mixing zone where the mixture is homogenous. CVS systems are quite large and costly, especially when they are designed for large flow rates.
The raw modal and partial flow sampling methods have proven to be adequate, but especially for transient test cycles they require a fast and accurate exhaust flow measurement system to control the proportionality of gaseous and particulate sampling. International standards like ISO 8178 or ISO 16183 specify exhaust gas flow measurement methods and accuracy requirements. The preferred type of system that may be employed to gather and test the exhaust is a direct vehicle exhaust flow measurement device (DVE). A DVE receives the entire volume of exhaust gases and measures the exhaust flow directly at the tailpipe. A wide variety of measurement principles has been applied, i.e. ultrasonic flow meters or differential pressure devices, like flow nozzles. An additional requirement for vehicle testing is a low pressure change at the exit of the tailpipe, which can only be achieved with a low flow restriction of the flow metering device.
Internal combustion engines utilize poppet valves to release the spent exhaust gases from the combustion cylinder to the tailpipe. As the exhaust valves open and close pressure pulsations are created within the exhaust gas stream. Emissions standards have become more stringent, and therefore, measurement accuracy has become increasingly important. The pressure pulsations within the exhaust gases may introduce an error into the flow measurement and may result in an inaccuracy in the emissions test. Precautions have to be taken or an active correction has to be employed to minimize the impact of these pressure pulsations on the flow measurement.
One embodiment of a DVE based on differential pressure devices is described in the prior art. This embodiment mixes the total amount of exhaust with make-up air and measures the mixed exhaust flow rate as well as the flow of make-up gas such as ambient air. The exhaust flow is calculated by subtracting the make-up air flow into the system measured by a subsonic venturi from the total flow of make-up air and exhaust gases through the system measured by a critical flow venturi. A correction factor might be generated by comparing the flow of make-up air through the subsonic venturi and the critical flow venturi during a calibration process in which the tailpipe is closed. However, an inaccuracy results with such a calibration procedure because of the nonlinear relationship between the differential pressure across the subsonic venturi and the flow rate through this device. Other inaccuracies are introduced by the presence of engine pulsations. Further, inaccuracies are caused in the temperature measurement from inhomogeneous mixing of make-up air and hot exhaust gases.
The composition of the make-up air used to dilute the sample exhaust gas is constant. However, the amount the sample exhaust gas is diluted during a given test varies. As a result, the density of the diluted exhaust gas varies throughout the test, which introduces an error in the measurements.
Typically, the emissions test takes place in a rather confined cell in which space is limited. As a result, the exhaust gas flow measurement system must be rather compact but without compromising the high level of accuracy needed to comply with the increasingly stringent emissions test standards. Therefore, what is needed is an exhaust flow measurement device with low flow restriction that may be packaged within a cell while providing an accurate flow measurement with reduced sensitivity to exhaust gas pressure pulsations.